import java.util.Scanner;

// Press Shift twice to open the Search Everywhere dialog and type `show whitespaces`,
// then press Enter. You can now see whitespace characters in your code.
public class Main {
    public static void main(String[] args) {
        Scanner reader = new Scanner(System.in);
        int n = reader.nextInt();
        
        double sum = 0.0;
        for(int i=1; i<=n; i++) {
            sum += 1.0/(1+i*i);
        }
        System.out.println(sum);
        System.out.println("程序结束");
    }
}